The present invention relates to a protection circuit module for a rechargeable battery, and also to a method of making the same.
A rechargeable battery pack, which may be provided with a lithium-ion cell, incorporates a protection circuit to prevent overcharge.
FIG. 5 shows a conventional protection circuit module used for such a protective purpose. It includes a circuit board 51 having a main board portion 51a and a protrusion 51b projecting from a corner of the main board portion 51a. The circuit board 51 carries conductive plates 52, 53 soldered thereto at their one ends. These plates will be electrically connected to the anode or cathode of a rechargeable lithium-ion battery (not shown) with or without an intermediate connector. The circuit board 51 is provided with a wiring pattern (not shown) and a terminal base 54. Various electronic parts are mounted on the board to constitute the protection circuit. The terminal base 54 supports conductive plates 54axcx9c54d serving as power input/output terminals through which power is taken in or out from the battery pack. The above-mentioned electronic parts include field-effect transistors 55a, 55b and a control IC (integrated circuit) 56 as semiconductor elements, and a plurality of passive elements 57axcx9c57k. The passive elements 57axcx9c57k may be a resistor, capacitor, or thermistor for example. The field-effect transistors 55a, 55b and the control IC 56 each have a resin package from which electrical leads project to be soldered to the circuit board 51.
The conductive plate 52 is connected to the conductive plate 54a via the wiring pattern on the circuit board 51, while the conductive plate 53 is connected to the conductive plate 54a via the wiring pattern on the board 51 and the field-effect transistors 55a, 55b mounted on the board 51.
Unfavorably, the conventional design has the following drawbacks since the transistors 55a, 55b and IC 56 mounted on the circuit board 51 are resin-packaged.
First, the on-resistance of the field-effect transistors 55a, 55b cannot be reduced to a desired level. Specifically, in the packaged components, use is made of a wire for bonding a bare chip to an electrical lead. Since the bonding wire has a certain resistance, the on-resistance may fail to be rendered satisfactorily small. Accordingly, the power loss fails to be restricted to a low level. One solution to address this problem may be to use a plurality of wires arranged in parallel for performing the wire-bonding. In this manner, though the on-resistance can be reduced to some extent, the production costs will unduly increase.
Second, the size of the resin package is much larger than that of a bare chip. Accordingly, the protection circuit module, and hence the battery pack, cannot be downsized to a desire level.
Third, in a typical protection circuit module, the component-mounted circuit board 51 as a whole is usually coated with protective resin for water resistance. However, since the field-effect transistors 55a, 55b and the control IC 56 are initially provided with a protective resin package of their own, the second resin coating may be superfluous, merely to incur an increase in the production cost.
Fourth, the resin-packaged field-effect transistors 55a, 55b and the control IC 56 are more expensive than the bare chips. Accordingly, the production cost of the conventional protection circuit module is made greater.
Additionally, in the conventional layout, the conductive plates 52 and 54a are electrically directly connected to each other. Despite of this, the conductive plate 52 is first soldered to the circuit board 51 and then connected to the conductive plate 54a via the wiring pattern formed on the circuit board 51. Due to this, extra room for fixing the conductive plate 52 or providing the wiring pattern is needed, whereby the circuit board is made unduly large. This hinders the size and weight reduction of the battery pack. Also, the reduction in production cost may not be readily achieved since one additional step is needed to attach the conductive plate 52 to the circuit board 51.
The present invention has been proposed for overcoming or alleviating the problem entailed in the above-noted conventional design. It is, therefore, an object of the present invention to provide a protection circuit module for a rechargeable battery whereby the on-resistance of a field-effect transistor is advantageously reduced, and whereby the thickness, overall size, weight and production costs of the rechargeable battery pack are also reduced. Another object is to provide a method of making such a protection circuit module.
According to a first aspect of the present invention, there is provided a protection circuit module for a rechargeable battery. The module includes: a circuit board; a predetermined number of semiconductor elements including at least a field-effect transistor mounted on the circuit board for charge and discharge; and a predetermined number of passive elements mounted on the circuit board. The semiconductor elements are mounted facedown as bare chips onto the circuit board.
Preferably, the semiconductor elements and the passive elements are enclosed by a protection coating.
According to a second embodiment, there is provided a method of making a protection circuit module for a rechargeable battery, where the module includes: a circuit board; a predetermined number of semiconductor elements including at least a field-effect transistor mounted on the circuit board for charge and discharge; and a predetermined number of passive elements mounted on the circuit board. Bare chips having solder bumps are used for the semiconductor elements, and these bare chips together with the passive elements are simultaneously mounted on the circuit board by solder reflowing.
According to the first and second aspects of the present invention, the bare chip semiconductor elements are mounted facedown on the circuit board. In this manner, the on-resistance of the field-effect transistor can be properly reduced. At the same time, it is possible to reduce the thickness, overall size and production cost of a rechargeable battery pack.
More specifically, since the bare chip of the field-effect transistor is directly mounted on the circuit board, the resistance of bonding wires is omitted. Thus, the on-resistance of the field-effect transistor is properly reduced, which is advantageous to reducing the power loss. In addition, without using no packages, the protection circuit module can be made thinner and smaller. Accordingly, the rechargeable battery pack is also made thinner and smaller. Further, the bare chip is less expensive than the packaged product. Therefore, the protection circuit module for a rechargeable battery can be produced at a reduced cost.
Further, use is made of bare chips with solder bumps for the semiconductor elements, and these bare chips together with passive elements are mounted on the circuit board by solder reflowing. In this manner, it is possible to enjoy the above-noted advantages resulting from the direct mounting of the field effect transistor bare chips on the circuit board, and also to perform the soldering procedure very effectively. As a result, the production efficiency is improved, while the production cost is reduced.
According to a third embodiment of the present invention, there is provided a protection circuit module for a rechargeable battery. This module includes: a circuit board; a first conductive plate attached to the circuit board to be electrically connected to the rechargeable battery; a second conductive plate attached to the circuit board to be electrically connected to the first conductive plate; and a terminal base at which the second conductive plate is disposed as a terminal. The feature may reside in that the first conductive plate and the second conductive plate are integrated into a single conductive plate.
Preferably, the conductive plate may have one end disposed on the terminal base and another end projecting outward from the circuit board.
According to the third aspect of the present invention, use is made of a conductive plate integrating the first and the second conductive plates. Therefore, there is no need to provide extra room for fixing the first conduct plate to the circuit board or for forming the wiring pattern to connect the first and the second conductive plates. Accordingly, the circuit board is made smaller, and hence the size reduction or weight reduction of the rechargeable battery pack can be achieved. Further, no first conductive plate needs to be attached to the circuit board, which facilitates the reduction in production cost.
According to a fourth aspect of the present invention, there is provided a protection circuit module for a rechargeable battery. The module includes: a circuit board; a predetermined number of semiconductor elements including at least a field-effect transistor mounted on the circuit board for charge and discharge; a predetermined number of passive elements mounted on the circuit board; a first conductive plate attached to the circuit board to be electrically connected to the rechargeable battery; a second conductive plate attached to the circuit board to be electrically connected to the first conductive plate; and a terminal base at which the second conductive plate is disposed as a terminal. The first conductive plate and the second conductive plate are integrated into a single conductive plate. Also, the semiconductor elements are mounted facedown as bare chips onto the circuit board.
With such an arrangement, it is possible to enjoy the above-mentioned advantages resulting from the first and the third aspects of the present invention.